The present invention relates to the non-contact measurement of the sheet resistance and leakage current of p-n junctions.
Advances in semiconductor technology have increased the requirements in monitoring epi and ion implant sheet resistance, Rs, in the range of 50 to greater than 10,000 Ohms/square, and also for the measurement of leakage current of p-n-junctions.
Currently, 4-point probe (4PP) techniques are widely used for sheet resistance measurements. In the case of ultra-shallow p-n junctions (with junction depths less than 30 nm) this technique has several disadvantages: mechanical probes can penetrate through the implant layer; and probe pressures necessary for making ohmic contact with an implant layer can increase p-n junction leakage between the implant layer and the underlying opposite conductivity substrate; also if a p-n junction is leaky, a 4PP probe can give erroneous values for sheet resistance even without probe penetration.
Various photo-electrical techniques have been proposed for non-contact measurement of p-n junctions which have limitations in the range of sheet resistance values which can be measured as well a numerous technical limitations on measurement accuracy and calibration.
The present invention uses a non-contact junction photovoltage (JPV) technique for measurement of the sheet resistance and leakage current. JPV is the change of the near surface band bending or surface barrier under intensity modulated illumination of a p-n junction.
A non-contact JPV technique for measurement of sheet resistance in ultra shallow p-n junctions was proposed in Roger L. Verkuil, U.S. Pat. No. 5,442,297. This technique is based on the measurement of JPV signals remote from a local illumination area. To detect the attenuation and phase monitoring, the apparatus included two conducting rings placed in the vicinity of the wafer surface outside the illumination area. Using the measurement of two AC signals outside the illumination region and additional junction capacitance data, the sheet resistance can be calculated.
This technique has the following disadvantages. Since only attenuated JPV signals are measured outside the illumination area, this approach does not provide good spatial resolution and high sensitivity for measurements of sheet resistance Rs<400 Ohms/square in ultra shallow p-n junctions formed with high dose implants. The measurement is based on a small signal, linear JPV theory. According to this theory, the JPV signal should be linear with light intensity not only outside of the illumination area but also inside this area. The technique presented in U.S. Pat. No. 5,442,297 uses measurements only outside the illumination area. The calculation of sheet resistance is based on a simplified model valid only for infinitely thin metal rings electrodes. As a result, this model will give additional systematic errors because the capacitance of these thin electrodes depends upon non-linear fashion on their distance from the wafer surface and the linearity condition is not checked within the illumination area. This probe configuration does not allow one to produce accurate measurements close to the edge of the wafer. Another disadvantage of this patent is the requirement for the use of additional means for measurements of the p-n junction capacitance, or information about this value in order to calculate sheet resistance values. Also this technique does not provide for measurement of p-n junction conductance (leakage)—an important parameter for USJ process.
The advantages of present invention are a method and apparatus for accurate measurements of sheet resistance, capacitance of p-n junctions and determination of leakage currents with improved spatial resolution and sensitivity.